Method for hydrogenating aromatic nitro compounds

ABSTRACT

A method for hydrogenating aromatic nitro compounds by reacting at least one aromatic nitro compound with hydrogen in two adiabatically operated fixed-bed catalytic reactors arranged in series, wherein one part of the reaction mixture from the first reactor is recirculated therein while the other part is fed into the second reactor.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR96/01893, filed on Nov. 29, 1996.

The present invention relates to the hydrogenation of aromatic nitrocompounds using a catalyst in a fixed bed.

BRIEF DESCRIPTION OF DRAWING

Among the processes of this type, two categories can be listed, thefirst using a catalyst in the form of a stirred suspension, the secondusing a catalyst in a fixed bed.

Processes involving catalysts in the form of a stirred suspension haveexperienced considerable industrial developments. The reason for this isthat, despite a large investment, they have the advantage of effectivelycontrolling the exothermicity of the hydrogenation reaction, which isconsiderable for compounds of this type. Moreover, the hydrogenation ofnitro compounds on a catalyst in a stirred suspension exhibits goodperformance, in terms of selectivity of desired compound, conversion ofthe nitro product and production efficiency. Lastly, the catalyst isrelatively easy to replace for these processes, which contributestowards conserving their good performance.

The other category consisting in using catalysts in a fixed bed tohydrogenate aromatic nitro compounds has not experienced as extensive anindustrial-scale development as the previous category. This is becausethe major difficulty with this process lies in the removal of the heatevolved during the reaction; if this removal of heat is not sufficient,it can lead to a runaway reaction, and also to a lowering in theperformance of the process and degradation of the catalyst used.

One solution recommended to remove the heat of reaction has been torecycle a fraction of the reaction mixture, which has been cooledbeforehand.

Thus, it has been proposed to hydrogenate aromatic nitro compounds in atubular reactor with a fixed bed of flowing type (i.e. injection of thereactants co-currentwise downwards in the reactor) in which the supplyflow comprises some of the recycled reaction mixture coming from thesaid reactor. However, the reactor dimensions are such that this reactorcannot be considered as operating adiabatically since 60 to 70% of theheat exchanges take place by means of losses through the walls of thesaid reactor. Consequently, extrapolation of this process to theindustrial scale would result in having reactors whose height would beconsiderably greater than their cross-section, so as to conserve areasonable level of heat exchange through the walls. However, suchreactors would need to use very large amounts of catalyst, which woulddetract from one of the advantages of the process with a fixed bedcompared with catalyst suspensions. Another possibility for theindustrial-scale exploitation of this type of process would be toincrease the heat exchanges with the recycled reaction flow. In otherwords, this would consist in increasing the rate of recycling of thereaction medium considerably. In this case, the production efficiency ofsuch a process would be much too low, on account of the large dilutionrequired for the compounds to be nitrated and the need to convert thesecompounds completely.

As may be observed, there is currently no process for the fixed-bedhydrogenation of aromatic nitro compounds which can be industriallyexploited satisfactorily.

The subject of the present invention is thus to propose such a type ofprocess, which does not have the abovementioned drawbacks ofhydrogenation reactions using catalysts in a fixed bed.

Thus, the process according to the invention has all the advantages ofprocesses using fixed beds when compared with processes using a stirredsuspension of catalyst.

In particular, the process according to the invention is easy to carryout and incurs lower capital costs since it is no longer necessary toprovide complex systems for separating out the catalyst once thereaction is complete, nor even to provide stirring systems, with all theproblems of leaktightness inherent therein.

Furthermore, the process according to the invention allows thedifficulties associated with carrying out the reaction in a fixed bed,i.e. the exothermicity of the reaction, to be solved.

Lastly, the performance levels of the process according to theinvention, in terms of production efficiency and selectivity, areentirely comparable, in terms of production efficiency and selectivity,with those obtained by processes using stirred catalyst suspensions.

These aims and others are achieved by the present invention, the subjectof which is thus a process for hydrogenating aromatic nitro compounds,in which at least one aromatic nitro compound is reacted with hydrogenin two fixed-bed catalytic reactors in series operating adiabatically;in this process, some of the reaction mixture leaving the first reactoris recycled into this reactor, the other part being introduced into thesecond reactor.

However, other characteristics and advantages of the present inventionwill become apparent on reading the description, the figure and theexamples which follow.

For reasons of convenience, the apparatus units and their assembledsequence will first be described.

As has been mentioned previously, the process according to the inventionis carried out in an assembly of two reactors in series.

Each of the reactors is preferably supplied co-currentwise with gas andliquid. It is possible to inject the reactants and/or the reactionmixture into the top or the bottom of the reactor.

In the first case, the process is in a so-called "flowing" catalytic bedand the continuous phase in the reactor is a gaseous phase.

According to a particularly advantageous and preferred embodiment of theinvention, each of the reactors is supplied with hydrogen, at least onearomatic nitro compound and/or the recycled or non-recycled reactionmixture, co-currentwise, such that the continuous phase in the saidreactors is liquid.

Such a result is obtained by supplying the reactors from the bottomupwards. In this way, the catalyst bed is immersed in the reactantsand/or the reaction mixture.

This method of supply is particularly appropriate since it increases theefficiency of the gas-liquid and liquid-solid contacts. Moreover, thefact that the catalyst is continuously immersed reduces the risk of itdegrading due to local overheating.

It would not constitute a departure from the context of the presentinvention if two reactors supplied differently were used. However, it ispreferred to use two reactors supplied via the bottom.

One of the essential characteristics of the invention lies in the factthat some of the reaction mixture coming from the first reactor isrecycled into the supply flow of this reactor, the other part beingintroduced into the second reactor.

More particularly, the ratio of the flow rate of supply of the firstreactor with aromatic nitro compounds to the flow rate of reactionmixture recycled into the first reactor is such that the concentrationof nitro compounds is between 0.1 and 10% by weight in the firstreactor.

According to a specific embodiment of the invention, the ratio of theflow rate of supply of the first reactor with aromatic nitro compoundsto the flow rate of reaction mixture recycled into the first reactor isbetween 5 and 1000.

The temperature at the foot of the first reactor is about 50 to 200° C.and more particularly about 80 to 170° C.

On leaving the first reactor, the rise in temperature, i.e. thedifference between the outlet temperature of the reaction mixture andthe inlet temperature of the nitro compounds and/or of the recycledreaction mixture, is less than 120° C.

The mixture leaving the first reactor comprises unreacted nitrocompounds, the corresponding hydrogenated aromatic compounds, water andhydrogen.

According to one variant of the invention, the reaction mixture comingfrom the first reactor is introduced into a gas-liquid separator.

The resulting gaseous fraction is fed into the second reactor, whereasthe liquid fraction is separated into two parts, one of which isrecycled into the supply flow of the first reactor.

This separation operation can be carried out in any conventional type ofapparatus, such as cylindrical tanks or cyclones.

Prior to the recycling in the first reactor, the reaction mixture iscooled in order to have the desired temperature after mixing with thereactants.

Preferably, the reaction mixture leaving the first reactor is separatedfrom the gases it contains, prior to being recycled into this reactorand introduced into the second reactor.

Moreover, according to a specific embodiment of the invention, thereaction mixture introduced into the abovementioned exchanger is areaction mixture which has been freed of most of the gases it contains.

The reactors in which the process is carried out are preferablycylindrical tubes fitted with standard means for retaining the fixed bedof catalyst and for distributing the liquid and gas flows.

As may be observed, the reactors used are of simple and inexpensivetechnology, which represents an advantage over reactors with asuspension of stirred catalyst.

On account of the high level of recycling, the behaviour of the firstreactor is of the stirred type.

As regards the second reactor, a piston-type reactor is used. It isadvantageous to combine the two reactors since this makes it possible tominimize the overall amount of catalyst and thus the size of thereactors.

The size of these apparatus units can be determined in a conventionalmanner by those skilled in the art depending on the type of reactor(stirred, piston), the desired conversion and the flow rate of nitrocompounds to be hydrogenated.

Most of the hydrogenation reaction is carried out in the first reactor.A degree of conversion of greater than or equal to 90% of hydrogenatedaromatic nitro compounds relative to the aromatic nitro compounds fedinto the recycling loop is achieved.

The second reactor corresponds to a finisher by means of which degreesof conversion of 100% are achieved.

Aromatic nitro compounds processed according to the process of theinvention comprise at least one nitro group.

More particularly, the processed aromatic nitro compounds comprise atleast two nitro groups.

As aromatic nitro compounds which are particularly suitable for theinvention, mention may be made of nitrobenzene, mononitrotoluenes,dinitrobenzenes and dinitrotoluenes.

The process according to the invention can advantageously be carried outwith a mixture of isomers.

The aromatic nitro compounds can be used in the presence of a solventfor the said aromatic nitro compounds.

According to a first variant, the solvent used is chosen from aliphaticalcohols and cyclic ethers, alone or as a mixture.

More particularly, methanol, ethanol, propanol or isopropanol is used asaliphatic alcohol, alone or as a mixture.

As cyclic ether, mention may be made of dioxane or tetrahydrofuran,alone or as a mixture.

According to this variant, prior to injection into the first reactor,the fresh aromatic nitro compounds are dissolved in the abovementionedsolvent or mixture of solvents.

The concentration of nitro compounds in the solvent or mixture ofsolvents can vary within a wide range. However, single-phase mixtures,i.e. mixtures for which the solubility limit of the nitro compound(s) isnot reached for the solvent or the mixture in question, are used.Without wishing to be limited, the concentration range is less than orequal to 25% by weight.

According to a second preferred variant of the invention, the solventused is the reaction mixture obtained from the first reactor. If thisvariant is adopted, the fresh aromatic nitro compounds are supplied inthe liquid or molten state.

This variant is advantageous in the sense that it does not dilute thereactants, as is the case when a solvent or a mixture of solvents isused. This contributes in particular towards conserving a high processproduction efficiency. Moreover, this variant dispenses with anyadditional step of separation of the products or of the solvent(s) usedduring the reaction, which involves risks of losses of hydrogenatedproducts.

The hydrogen used is more particularly pure hydrogen. The term pure isunderstood to mean a gas comprising at least 99% hydrogen, and moreparticularly at least 99.9% hydrogen. It should be noted that it wouldnot constitute a departure from the context of the present invention ifthe hydrogenation reaction was carried out with dilute hydrogen,although this does not bring any particular advantages to the reaction.

The hydrogen is supplied in the stoichiometric amount. However, thehydrogen is preferably used in excess relative to the aromatic nitrocompounds.

More particularly, the hydrogen content is such that the excess relativeto the stoichiometry is between 5 and 50 mol %. It should be noted thatthe fact that the hydrogenation reaction is carried out in the presenceof an excess of hydrogen does not pose a problem of loss of this gassince it can be recycled.

The hydrogen pressure in the reactor ranges between 5 and 150 bar,preferably between 10 and 50 bar.

This gas is supplied by any means known to those skilled in the artwhich makes it possible to have homogeneous distribution of this gasinside the reactor.

The reaction is carried out in the presence of a hydrogenation catalystwhich is conventional in this field.

Thus, a catalyst based on a metal from group VIII of the Periodic Tableof the Elements, which may or may not be supported, is used.

Among the metals of group VIII, mention may be made most particularly ofnickel, platinum and palladium, these metals being used alone or as amixture.

Among the unsupported catalysts, Raney nickel is particularly suitable.

When the hydrogenation is carried out in the presence of a supportedcatalyst, the support can be chosen from inert materials such askieselguhr, charcoal, alumina and silica.

In this case, the metal content is usually between 0.1 and 10% byweight, preferably between 0.3 and 5% by weight.

The catalyst is in a form which is suitable for use in a fixed bed. Forexample, beads, extrudates and chips are possible forms.

The objects are generally between 1 and 5 mm in size.

The figure represents a preferred embodiment of the invention, i.e. onein which the catalytic bed is immersed. The flows (1) and (2)respectively represent the hydrogen and the aromatic nitro compound,which is preferably in the molten state. The flow (1) is homogeneouslydistributed inside the reactor.

The flows (1) and (2) are placed in contact with the catalyst in thereactor A.

On leaving the reactor, the flow (3) is introduced into a gas-liquidseparator B from which a gaseous fraction (4) mainly comprisinghydrogen, and a liquid fraction (5) are separated.

According to a first embodiment, all of the flow (5) is conveyed to theexchanger C. The aim of this exchanger is to lower the temperature ofthe degassed reaction mixture so as to obtain the desired temperatureafter mixing with the nitro compounds in the flow (2).

On leaving the exchanger C, some of the flow (5a) is conveyed to thereactor D; the other part, (5b), is recycled into the foot of thereactor A.

According to a second embodiment, some of this flow (5) comes fromupstream of the exchanger C. This makes it possible in particular toreduce the size of the exchanger. Thus, according to this embodiment,some of the flow referred to as (5'a) and/or some of the flow referredto as (5'b) can come from upstream of the exchanger, in order then tocombine them with the flows (5a) and (5b) respectively.

The liquid fraction (5a) or (5'a) and the gaseous flow (4) feed thereactor D so as to keep the catalytic bed immersed. The reaction mixture(6) is introduced into a separator and the gaseous fraction (7), mainlycomprising hydrogen, can advantageously be recycled upstream in theprocess, for example into the supply of the reactor A. The liquidfraction (8) comprising the water and the hydrogenated aromaticcompounds are processed in order to remove the water.

The hydrogenated compounds can be used in particular for the preparationof isocyanates, which are polyurethane intermediates.

The example which follows illustrates the invention without limiting it.

EXAMPLE

The hydrogenation reaction is carried out in apparatus whose fittingscorrespond to those of the figure.

Reactor A corresponds to a tube 45 mm in diameter and 800 mm in height.

Reactor D corresponds to a tube 15 mm in diameter and 500 mm in height.

Reactor A comprises 643 g of a Pd/C catalyst comprising 0.5% by weightof palladium, in the form of chips with an average size of about 3 mm.

Reactor D comprises 50 g of the same catalyst as above.

Industrial dinitrotoluene comprising 96% of a mixture of 2,4- and2,6-isomers in a 4:1 ratio and 4% of 3,5-/3,4-/2,5-isomers areintroduced at a flow rate of 245 g/h.

The hydrogen flow rate is 580 Nl/h.

The pressure in the reactor is 25 bar.

The inlet temperature in this reactor is 80° C. and the outlettemperature is 100° C.

A flow rate of 20 l/h of degassed reaction mixture cooled in order toobtain a temperature in the region of 80° C. is recycled into thereactor A.

In this reactor, the degree of conversion of the dinitrotoluene suppliedis greater than 95%.

Reactor D is fed such that the volume in the first reactor remainsconstant.

The pressure in reactor D is identical and the inlet temperature is 80°C.

On leaving this second reactor, the degree of conversion of thedinitrotoluenes is total and the selectivity is greater than 98%.

What is claimed is:
 1. A process for hydrogenating aromatic nitrocompounds through a hydrogenation reaction, comprising the steps of:a)reacting at least one aromatic nitro compound With hydrogen in twofixed-bed catalytic reactors in series and comprising a first reactorand a second reactor operating adiabatically to obtain a reactionmixture; b) recycling a part of the reaction mixture leaving the firstreactor into this first reactor, and introducing the other part into thesecond reactor; and c) supplying each of the reactors with hydrogen, atleast one aromatic nitro compound, or the recycled or non-recycledreaction mixture, co-currentwise, in order to obtain a continuous liquidphase in said reactors.
 2. A process for hydrogenating aromatic nitrocompounds through a hydrogenation reaction, comprising the steps of:a)reacting at least one aromatic nitro compound with hydrogen in twofixed-bed catalytic reactors in series comprising a first reactor and asecond reactor operating adiabatically to obtain a reaction mixture; b)recycling a part of the reaction mixture leaving the first reactor intothis first reactor, and introducing the other part into the secondreactor; and c) supplying each of the reactors with hydrogen, at leastone nitro compound, and the recycled or non-recycled reaction mixture,co-currentwise, in order to obtain a continuous liquid phase in saidreactors.
 3. A process according to claim 1, wherein the aromatic nitrocompounds present a concentration of between 0.1 and 10% by weight inthe first reactor.
 4. A process according to claim 1, further comprisingthe step of carrying out the reaction with the reaction mixture havingan outlet temperature and the aromatic nitro compounds or the recycledreaction mixture having an inlet temperature, the difference betweensaid outlet and said inlet temperature being less than 120° C.
 5. Aprocess according to claim 1, further comprising the step of separatinggases from the reaction mixture leaving the first reactor beforerecycling into this first reactor and before introduction into thesecond reactor.
 6. A process according to claim 1, wherein said aromaticnitro compounds present at least one nitro group.
 7. A process accordingto claim 1, further comprising the step of carrying out the reaction inthe presence of a solvent for said aromatic nitro compounds.
 8. Aprocess according to claim 7, wherein the solvent is an aliphaticalcohol or a cyclic ether.
 9. A process according to claim 1, furthercomprising the step of carrying out the reaction in the presence of anexcess of hydrogen, relative to the stoichiometry, of 5 to 50 mol %. 10.A process according to claim 1, further comprising the step of carryingout the reaction in the presence of a catalyst comprising at least onemetal from group VIII, optionally supported.
 11. A process according toclaim 2, wherein the aromatic nitro compounds present a concentration ofbetween 0.1 and 10% by weight in the first reactor.
 12. A processaccording to claim 2, further comprising the step of carrying out thereaction with the reaction mixture having an outlet temperature and thearomatic nitro compounds and the recycled reaction mixture having aninlet temperature, the difference between said outlet and said inlettemperature being less than 120° C.
 13. A process according to claim 2,further comprising the step of separating gases from the reactionmixture leaving the first reactor before recycling into this firstreactor and before introduction into the second reactor.
 14. A processaccording to claim 2, wherein said aromatic nitro compounds present atleast one nitro group.
 15. A process according to claim 2, furthercomprising the step of carrying out the reaction in the presence of asolvent for said aromatic nitro compounds.
 16. A process according toclaim 15, wherein the solvent is an aliphatic alcohol or a cyclic ether.17. A process according to claim 2, further comprising the step ofcarrying out the reaction in the presence of an excess of hydrogen,relative to the stoichiometry, of 5 to 50 mol %.
 18. A process accordingto claim 2, further comprising the step of carrying out the reaction inthe presence of a catalyst comprising at least one metal from groupVIII, optionally supported.
 19. A process according to claim 7, whereinthe solvent is the reaction mixture.
 20. A process according to claim 1,wherein the first reactor has a foot having a temperature of about 50 toabout 200° C.
 21. A process according to claim 19, wherein thetemperature is of about 80 to about 170° C.